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Conjunctive queries without distinguished variables are called boolean conjunctive queries.Conjunctive queries where all variables are distinguished (and no variables are bound) are called equi-join queries, [1] because they are the equivalent, in the relational calculus, of the equi-join queries in the relational algebra (when selecting all columns of the result).
A range query is a common database operation that retrieves all records where some value is between an upper and lower boundary. [1] For example, list all employees with 3 to 5 years' experience. Range queries are unusual because it is not generally known in advance how many entries a range query will return, or if it will return any at all.
The natural join can be simulated with Codd's primitives as follows. Assume that c 1,...,c m are the attribute names common to R and S, r 1,...,r n are the attribute names unique to R and s 1,...,s k are the attribute names unique to S. Furthermore, assume that the attribute names x 1,...,x m are neither in R nor in S.
SQL includes operators and functions for calculating values on stored values. SQL allows the use of expressions in the select list to project data, as in the following example, which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.
Andrew Yao showed [3] that there exists an efficient solution for range queries that involve semigroup operators. He proved that for any constant c, a pre-processing of time and space () allows to answer range queries on lists where f is a semigroup operator in (()) time, where is a certain functional inverse of the Ackermann function.
The first is whether a variable-column pair t.a is bound to the column of a relation or a constant, and the second is whether two variable-column pairs are directly or indirectly equated (denoted t.v == s.w). For deriving boundedness we introduce the following reasoning rules: in " v.a = w.b" no variable-column pair is bound,
In a relational database, a column is a set of data values of a particular type, one value for each row of a table. [1] A column may contain text values, numbers, or even pointers to files in the operating system. [ 2 ]
The EXCEPT ALL operator does not remove duplicates, but if a row appears X times in the first query and Y times in the second, it will appear (,) times in the result set. Notably, the Oracle platform provides a MINUS operator which is functionally equivalent to the SQL standard EXCEPT DISTINCT operator. [3]